"Translating librettos is a time-honored practice that takes enormous skill. It’s not like translating “Madame Bovary” into English, which is hard enough. A translation of an opera libretto must fit exactly the rhythm, bounce and flow of the existing melodic lines, which the composer matched to the words of the original language. Libretto translators are forced to play fast and loose with the meaning of the original text to render an equivalent in performable English."
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Translating Operas Into English Requires Tradeoffs - NYTimes.com